Identifying thymine in DNA base pairing

Steps:

• Examine the diagram for base pairs: adenine (A) pairs with thymine (T), guanine (G) with cytosine (C).
• Identify the strand showing a base opposite adenine, indicating thymine.
• Confirm thymine as a pyrimidine base with a methyl group, distinct from others.
• Match the label to the structure matching thymine's double-ring pyrimidine form.

Why D is correct:

• Thymine pairs specifically with adenine via two hydrogen bonds, as per Watson-Crick base pairing rules in DNA.

Why the others are wrong:

• A: Adenine is a purine that pairs with thymine, not matching a pyrimidine structure.
• B: Guanine is a purine that pairs with cytosine, forming three hydrogen bonds.
• C: Cytosine is a pyrimidine that pairs with guanine, not adenine.

Final answer: D